Commutators of known d.c. motors comprise at least three segments arranged to provide a substantially cylindrical outer surface and, according to conventional practice, this outer surface is made with a small value of roughness so as to avoid excessive frictional resistance to relative movement between the commutator and the brushes.
Regardless of the adequacy of this surface finish for most motor applications, it becomes inadequate under circumstances where the motor is fed with low voltages, has a small developed torque and has a starting torque specified for every angular position of rest.
A brush for low voltage applications must of necessity contain a large metallic porportion. This is usually a noble metal to avoid excessive oxidation. Silver is commonly used.
The brush to commutator interface influences shaft output starting torque in two ways.
Firstly, a resistance to mechanical motion, sometimes called stiction, is formed as a mechanical bond between the two components and requires fracturing before a sliding action can begin. The torque required to make this fracture is deducted from the torque developed by the armature, thus lessening the output torque to the shaft. The strength of the bond is a function of the area of surface contact, the type of metal in each component and the surface finish of the two components.
If silver from the brush is transferred to the commutator and some electrical polishing of this surface occurs due to electrical arcing, the result is two smooth surfaces of like metals in which atomic bonding readily takes place causing high values of stiction.
The second way in which the interface influences starting torque is concerned with the electrical resistance provided at the interface. If the interface surfaces are too smooth then the current densities in the large number of asperites in contact is too low to effectively burn away oxide films. The result is an increase in static resistance and an equivalent reduction in current and starting torque. Such an interface develops when silver is transferred to the commutator and electrically polished.